Information may be more rapidly processed and transmitted using optical as opposed to electrical signals. Optical signals can be used to enhance the performance of electronics processors. For example, electronic wires interconnecting integrated circuits (ICs) can be replaced with optical interconnects and the information processed with IC driven electro-optic modulators. Optical signals in fiber optic communications can be encoded on the optical carrier using electro-optic (EO) modulators. In both of these processes, nonlinear optical materials with second-order nonlinear optical activity are necessary to effect modulation of the light signal.
Nonlinear optical materials can also be used for frequency conversion of laser light. Such a conversion is desirable in many applications. For example, optical memory media are presently read using 830 nm light from diode lasers. The 830 nm light wavelength limits the spot sizes which can be read and hence the density of data stored on the optical memory media. Similarly, in fiber optic communications, light wavelengths of 1.3 .mu.m or 1.5 .mu.m are desirable due to the low transmission losses of glass fiber at those wavelengths. However, those wavelengths are too long for detection by Si based detectors. It is desirable to frequency double the 1.3 .mu.m or 1.5 pm wavelengths to 650 nm or 750 nm wavelengths where Si based detectors could be used.
Nonlinear optical materials which have been used in electro-optic devices have in general been inorganic single crystals such as lithium niobate (LiNbO.sub.3) or potassium dihydrogen phosphate (KDP). More recently, nonlinear optical materials based on organic molecules, and in particular polar aromatic organic molecules have been developed.
Organic nonlinear optical materials have a number of potential advantages over inorganic materials. First, organic nonlinear optical materials have higher NLO activity on a molecular basis. Organic crystals of 2-methyl-4-nitroaniline have been shown to have a higher nonlinear optical activity than that of LiNbO.sub.3. Second, the nonlinear optical activity of the organic materials is related to the polarization of the electronic states of the organic molecules, offering the potential of very fast switching times in EO devices. The time response of the organic nonlinear optical system to a light field is on the order of 10 to 100 femtoseconds. In contrast, a large fraction of the second order polarizability in the inorganic crystals in EO applications is due to nuclear motions of the ions in the crystal lattice, slowing the time-response of the materials. In addition, the low dielectric constant of the organic materials (e.g., 2-5 Debye at 1 MHz) compared to the inorganic materials (e.g. 30 Debye at 1 MHz) enables higher EO modulator frequencies to be achieved for a given power consumption. Third, the organic materials can be easily fabricated into integrated device structures when used in polymer form.
EP 218,938 and U.S. No. 4,859,876 have used an approach of incorporating NLO active molecules into amorphous polymer host matrices for NLO media. The NLO molecules are incorporated into the host by blending. Such doped polymers have the advantages of being easily fabricated into thin films suitable for integrated optical devices. The media contain organic molecules (dopants) with nonlinear optical activity with the advantages discussed above. These films must be oriented to achieve a non-centrosymmetric alignment of the NLO ohromophores. Such alignment is usually achieved by the application of an electric field across the film thickness while the temperature of the polymeric blend is above or near its glass transition temperature (Tg). The polymer is then cooled with the field on to lock the oriented molecules in place. EP 218,938 discloses a number of polymer host materials, including epoxies, and many types of molecules which have NLO activity including azo dyes such as Disperse Red 1. It is known that an NLO active material such as azo dye Disperse Red 1, (4-[N-ethyl-N-(2-hydroxyethyl]amino-4-nitro azobenzene), may be incorporated into a host by simply blending the azo dye in a thermoplastic material such as poly(methylmethacrylate), as described in Applied Physics Letters 49(5), 4 (1986) and U.S. Pat. No. 4,859,876.
While the doped polymer approach offers some advantages over organic and inorganic crystals, the approach has a number of problems. First, the stability of the NLO activity over time of such materials has been shown to be poor. A problem associated with a polymer with NLO properties produced by simply blending NLO molecules into a host polymer is that these polymer materials lack orientational stability. There is significant molecular relaxation or reorientation within a short period of time resulting in a loss of NLO properties. For example, as reported by Hampsch et al., Macromolecules 1988, 21, 528-350, the NLO activity of a polymer with NLO molecules blended therein decreases dramatically over a period of days at room temperature.
In addition, the NLO dopants in the blended polymeric media plasticize the polymer host matrix, lowering the polymer glass transition temperature (Tg). Lowering the polymer Tg has the effect of lowering the temperature stability of the electrically oriented NLO material or NLO medium. Near the Tg, segments of the polymer become mobile and the NLO active dopant molecules which were oriented electrically undergo orientational relaxation. Once orientational relaxation has occurred, the NLO medium exhibits no NLO activity.
A third problem with the doped polymers is the poor solubility of the NLO chromophore in the host matrix. Finally, the NLO chromophores tend to aggregate at relatively low doping levels (e.g., 5-20 percent w/v). Such aggregates scatter light and reduce the transparency of the waveguides to unacceptable levels.
Another disadvantage is that the polymer employed may have a low glass transition temperature, lack sufficient tensile strength, or other desirable properties for optical devices.
Japanese laid open publication Nos. J-63-275,553 and J-62-210,431 disclose various organic nonlinear optical compounds containing hydrazone functionalities which are useful for NLO applications. Specifically, J-62-210,431 discloses nonlinear optical materials containing nonlinear optical hydrazones as powders, molecule inclusions within the host lattice, thin layers deposited upon carriers such as films, monocrystals, and solutions. The hydrazones of J-62-210,431 may be bonded in the form of a pendant group to a polymer such as a polydiacetylene, but are not rigidly divalently bonded so as to form the backbone of the polymer.
It would be highly desirable to have organic polymeric materials which provide a rigid backbone comprising aryl hydrazone structural units as part of the backbone of the polymer.
It would also be desirable to provide organic polymeric materials with larger second and third order nonlinear optical properties than presently used organic electrolytic materials.
It is an object of this invention to make arylhydrazones that exhibit optical activity and are polymerizable with other commoners. It is further the object of this invention to obtain optically transparent polymers incorporating divalent moieties of the arylhydrazone structures which exhibit NLO activity upon orientation. It is an additional object of this invention that the polymers comprising the NLO materials or medium have a relatively high glass transition temperature. A high glass transition temperature will correlate with high temperature stability of the NLO material or medium.